Some plastic materials such as polymethylterpene and polysulfone are used for making cooking ware which is used in microwave ovens or electronic oven-ranges and component parts of electronic oven-ranges.
The methods of heating in the electronic oven-ranges are exemplified by range heating with the irradiation of microwaves and oven heating with hot air blasting and grill heating with infrared lamps. The cooking ware used in electronic oven-ranges and several component parts of electronic oven-ranges are required to have various properties such as excellent heat resistance, chemical resistance and desirable external appearance. Especially, concerning the heat resistance, very severe characteristic property is required because the parts or cooking ware are exposed not only to the high-frequency heat but also to the heat of oven heating or grill heating. For example, the temperature in an electronic oven-ranges is generally about 260.degree. C. and, in the case of oven heating, the temperature becomes partially about 300.degree. C. and, in the case of grill heating, the temperature sometimes becomes above 300.degree. C. Accordingly, the articles used for this purpose must not be deformed and must have sufficient mechanical strength in practical uses.
The wholly aromatic polyester has excellent properties due to its structure. Especially, in view of the heat resistance, it is the best one among all sorts of resins. The wholly aromatic polyesters which are prepared from terephthalic acid, isophthalic acid, p-hydroxy benzoic acid or its derivatives, and biphenyl-4,4-diol (4,4-diphenol) or its derivatives are widely used for electric and electronic fields such as ovens and ranges because they can be shaped by injection molding, their mechanical and electrical properties are excellent and they can meet other various requirements in the use of plastic articles such as good heat resistance, chemical resistance, oil resistance, radiation resistance, and dimensional stability (e.g. Patent Publication No. Hei 4-20327).
The above-mentioned wholly aromatic polyesters are acceptable in the heat resistance as represented by heat distortion temperature. It is, however, pointed out as a defect that, when injection molded articles are put in an atmosphere of 300.degree. C. or above, oven-blistering (blistering caused by heat) is liable to occur in the surfaces of molded articles. When oven-blistering occurs in the surface of molded articles, the external appearance of them are spoiled resulting in the loss of their commercial value. The cause of this blistering is not always clear. Even in the case of highly heat resistant wholly aromatic polyester resin having a heat distortion temperature of above 350.degree. C., the blistering often occurs at temperatures below 300.degree. C. In other words, in the electronic oven-ranges having functions of oven heating and grill heating, when the wholly aromatic polyester is used as their constituent parts or as cooking ware, they are difficultly used because of the occurrence of blistering due to their inferior characteristics for the use in electronic oven-ranges.
In order to improve the oven-range resistance (to avoid blistering), oven-range parts or oven ware of molded articles are previously subjected to heat treatment at a temperature below the blistering temperature. However, this method is not practical because the production cost is increased with lowering the yield rate.
In recent years, it is proposed to apply the coating of fluorocarbon resin to surfaces of wholly aromatic polyester articles in order to impart non-stickiness and low frictional property molded articles of wholly aromatic polyester (e.g., Patent Publication No. Hei 4-20327). The fluorocarbon resins used for the coating, polyfluoroethylenes are most suitable in view of heat resistance. Among them, polytetrafluoroethylene is most desirable.
As a method for applying the polyfluoroethylene to the surface of molded articles, it is generally done that the dispersion or enamel of polyfluoroethylene is applied to the surfaces of articles by spraying, brush coating or else and the coated articles are heated to a temperature above the melting point of the polyfluoroethylene, which is flowed by baking.
The wholly aromatic polyesters are good in the heat resistance which is represented by the heat distortion temperature thereof. However, it is pointed out as a defect that, when injection molded articles are put in an atmosphere of 300.degree. C. or above, oven-blistering is liable to occur in the surfaces of molded articles. For this reason, when the wholly aromatic polyester articles are coated with polyfluoroethylene as described above, there are problems that blistering is often caused to occur in the process of heating the molded articles to temperatures of 300.degree. C. to 340.degree. C. or above and the frequency of blistering is increased resulting in the lowering of yield rate of products.
As the methods for avoiding the problem of this kind, it is considered to lower heat treatment temperature or to use fluorine-containing resin having a low boiling point as a coating resin material. However, the peeling off or rupturing of coating film in the use is liable to occur in the former case, while the use of molded articles at high temperatures is limited due to the low heat resistance of the coating in the latter case.
In another method for settling the above problem in order to improve the blistering resistance, the heat treatment of molded articles is done prior to the coating process. In this case, it is most effective that the heat treatment is carried out at a temperature below the melting point, for example, at a temperature which is lower than the melting point by 80.degree. C. It is proposed that a coating of polytetrafluoroethylene is formed on the surface of a plate. However, the introduction of such heat treatment process causes the serious rise of production cost and the lowering of productivity in the production site, which are not desirable.
In the case of a copolyester which is made by copolymerizing hydroxycarboxylic acid such as hydroxybenzoic acid, the sequences of structural units derived from the hydroxycarboxylic acid may become different with preparation conditions. In other words, with preparation conditions, the structural units derived from hydroxycarboxylic acid are introduced into the copolyester in a block form or in a random manner. It is easily supposed that the properties of obtained copolyesters are varied in accordance with the variation of structures. In the present status of art, however, it has not yet been clarified sufficiently what kind of sequence of copolyester has influence on properties such as fluidity, workability, heat resistance and blistering resistance. Besides the sequencing property of hydroxycarboxylic acid, in copolyesters of a plurality of both acids and diols, there is a possibility that the structural factor such as the arrangement of these acids and diols has influence on the physical properties of the copolyester. However, the influences of structural factor like this on physical properties have never been revealed sufficiently.
As described above, although the wholly aromatic polyester is excellent in heat resistance, the difficulty in moldability has hitherto been pointed out. Furthermore, although the heat distortion temperature thereof is high, it has been pointed out that the oven blistering is liable to occur in the molded articles e.g. injection molded articles.
In the conventional method to improve moldability, the melting point is made low by lowering the linearity or rigidity of polymer by means of the copolymerization of non-linear monomers such as isophthalic acid and 6-hydroxy-2-naphthoic acid; the copolymerization of monomers having soft chains such as ethylene glycol; and the copolymerization of monomers having bulky substituent groups such as chlorohydroquinone and phenylhydroquinone.
In these methods, however, because the improvement in moldability is attained only by the lowering of melting point, the heat resistance, especially the heat distortion temperature is largely lowered in comparison with the lowering of melting point. Therefore, it is quite difficult to balance the moldability with heat resistance.
Accordingly, it has been demanded to provide a wholly aromatic polyester which is good in fluidity and is balanced in moldability, heat resistance and blistering resistance.
The object of the present invention is to provide machine parts of electronic heating devices and oven ware which are excellent in mechanical strength, heat resistance, moldability and electronic oven-range resistance.
Another object of the present invention is to provide a fluororesin coating material using wholly aromatic polyester which can be used for applying polyfluoroethylene coating to the surfaces of molded articles without the necessity of pretreatment such as heat treatment.